Before commencing excavation or other work where electrical cables, fibre optic cables or other utilities ducts or pipes are buried, it is important to determine the location of such buried cables or pipes to ensure that they are not damaged during the work. It is also useful to be able to track a path of buried cables or pipes. Current carrying conductors emit electromagnetic radiation which can be detected by an electrical antenna. If fibre optic cables or non-metallic utilities ducts or pipes are fitted with a small electrical tracer line, an alternating electrical current can be induced in the tracer line which in turn radiates electromagnetic radiation. It is known to use detectors to detect the electromagnetic field emitted by conductors carrying alternating current.
One type of such detector works in one of three modes. These modes are classified as either passive or active modes, the passive modes being ‘power’ mode and ‘radio’ mode. Each mode has its own frequency band of detection.
In power mode, the detector detects the magnetic field produced by a conductor carrying an AC mains power supply at 50/60 Hz, or the magnetic field re-radiated from a conductor as a result of a nearby cable carrying AC power, together with higher harmonics up to about 3 KHz. In radio mode, the detector detects very low frequency (VLF) radio energy which is re-radiated by buried conductors. The source of the original VLF radio signals is a plurality of VLF long wave transmitters, both commercial and military.
In the active mode, a signal transmitter produces an alternating magnetic field of known frequency and modulation, which induces a current in a nearby buried conductor. The signal transmitter may be directly connected to the conductor or, where direct connection access is not possible, a signal transmitter may be placed near to the buried conductor and a signal may be induced in the conductor. The buried conductor re-radiates the signal produced by the signal transmitter.
These systems are widely available and have been marketed by Radiotection Ltd for some time under the trade marks C.A.T.™ and GENNY™.
This invention provides further advancements to existing systems, providing additional functionality and benefits to the user. The detector achieves good performance in terms of sensitivity, dynamic range and selectivity. Typical parameters are 6×10−15 Tesla sensitivity (referred to a 1 Hz bandwidth), 141 dB rms/√Hz dynamic range, and a selectivity which allows 120 dB attenuation across a 1 Hz transition band. The detector can be digitally programmed to receive any frequency up to 44 kHz and processed through any defined bandwidth.
According to the invention there is provided a detector for detecting a buried current carrying conductor, comprising: two magnetic sensors, each magnetic sensor for converting electromagnetic radiation from the conductor into a field strength signal; an analogue to digital converter arranged to convert the field strength signals into digital signals; and a digital signal processor arranged to process the digital signals and to isolate signals of predetermined frequency bands; wherein the digital signal processor is programmed to process the digital signals in their baseband.
Preferably the analogue to digital converter is a delta-sigma converter.
Preferably the analogue to digital converter is a stereo CODEC.
Preferably each of the magnetic sensors has a noise floor and the field strength signals are input to an amplifier, to lift the magnetic sensor noise floor above an intrinsic quantisation noise of the analogue to digital converter referred to the bandwidth of one or more of the predetermined frequency bands, the output of the amplifier being provided as an input to the analogue to digital converter.
Preferably there are no selective filtering means, multiple switch gain stages and/or a phase sensitive heterodyne circuit between the magnetic sensor and the analogue to digital converter.
Preferably the digital signal processor is programmed to process signals from each magnetic sensor in two or more frequency bands simultaneously, those bands being selected from: (i) very low frequency band; (ii) mains electricity frequency band; and (iii) a predetermined frequency band produced by a dedicated signal transmitter for a detector for detecting a current carrying conductor.
Preferably the digital signal processor is programmed to process signals from each magnetic sensor in three frequency bands simultaneously.